This invention relates to electrodes for electrolytic processes, the electrocatalysis of redox reactions and, more particularly, to outer film coverings on an electrode.
Electrochemical methods of manufacture are becoming ever increasingly important to the chemical industry due to their great ecological acceptability, potential for energy conservation and the resultant cost reductions possible. A great deal of effort has been applied to the hardware of such methods.
One major element of the hardware aspect is the electrode itself. In general, requirements of an effective electrode have included cost, stability, conductivity and electrocatalytic activity. An electrode cost within the range of commercial feasibility is desirable. Ordinarily, the electrode must be relatively stable toward its own oxidation or reduction by reactants, products and/or intermediates in an electrolytic cell. The conductivity of both the substrate and surface layers of the electrode is important from the viewpoint of minimizing voltage losses associated with ohmic drop across the surface and the formation of surface insulating layers. The electrocatalytic activity of the electrode for a selected reaction contributes toward lowering the potential and raising product selectivity in an electrolytic cell.
Only a few materials may effectively constitute an electrode, particularly as an anode, because of the susceptibility of most substances to intense corrosive conditions. Among the effective electrode materials are graphite, nickel, lead, lead alloy, platinum or platinized titanium. Electrodes of this type have limited application because of such disadvantages as a lack of dimensional stability, high cost, low chemical activity, contamination of the electrolyte, contamination of the cathode deposit, sensitivity to impurities, lack of selectivity for the desired reaction, and ineffective generation of current densities or high overpotentials when employed in cells. Ineffective current densities of electrodes employed in a large number of commercial electrolytic cells are ordinarily less than about 50 milliamps/square centimeter (mA/cm.sup.2), while overpotentials typically refer to the excess electrical potential over the theoretical potential at which the desired product is discharged at the electrode surface.
Also, there are processes in which the products of electrolysis should preferably not be contaminated with material emitted from electrodes during operative conditions. When such a material is present in the products of the electrolysis, it must be removed by a separate treatment.
Many attempts and proposals have been disclosed to overcome some of the problems associated with electrodes and electrolytic cells, none of which seem to have accomplished an optimization of the desirable characteristics for an electrode. Presently, there is considerable interest in the development of films that can be deposited upon electrodes in an effort to lower the overpotential of selected redox reactions. Numerous film coatings have been employed on the surfaces of such electrodes, such as manganese dioxide, lead dioxide and the oxides of the platinum group metals.
However, a need still exists for electrolytic cells employing inexpensive, yet stable electrodes containing a surface coating that has effective electrocatalytic properties for selected reactions in such cells.
Accordingly, it is an objective of the invention to provide an electrode which utilizes relatively inexpensive materials and which is nevertheless stable during electrolytic processes and, additionally, is electrocatalytic to a number of reactions, such as oxidation or reduction of organic molecules.
Another object of the invention is to provide processes for making an electrode from substrates having low electrocatalytic activity and processes utilizing the electrode, especially in electrolytic cells.
It is a further objective of the invention to provide an electrode comprising a highly conductive electrocatalytic coating material and capable of generating a significant current when immersed in an electrolytic solution.
A further object still is to provide a coating material for a stable electrode that substantially enhances the electrocatalytic activity of the electrode as compared to the same electrode in the uncoated form.
A still further object is to provide an electrolytic cell employing an electrode containing an electrocatalytic coating material and, more particularly, to provide a cell employing an electrode capable of generating significant current densities.
These and other objects and advantages of the invention will become apparent from the following description.